Whats quirky about it? One could plumb it to cycle water while driving thus wasting absolutely zero fuel be it gasoline, diesel or propane. Its ideal for the 'set it and forget it' type that don't want to have to pack
one more thing and plan to use them heavily. Even while using at a standstill I can heat 5 gallons for pennies worth of gasoline, something you can't do with the rising cost of throw-away propane cylinders that are not intended to be refilled. To compare the longevity of a engine heat exchanger with some of the others is nearly impossible, under normal (off-road use included) circumstances the heat exchanger will never wear out. Hence why they are used in the marine and industrial applications regularly. Its by not means the 'one size fits all' answer but to call it quirky is naive at best.
There is a reason a handful of Aussie companies make engine based heat exchangers for use all over the Outbacks of the world, they work